A dialysis system typically includes a hydraulic system for circulating blood, a hydraulic system for circulating dialysate fluid, and a semi-permeable membrane. The blood system and the dialysate fluid system have fluid flow paths that extend past the membrane on opposite sides of the membrane. Urea and other blood components, but not blood cells, travel across the membrane from the blood side to the dialysate side as the blood and dialysate fluid both flow past the membrane.
In the oldest type of dialysis systems, the dialysate fluid, which is water infused with minerals and other essential blood components, is continually directed to flow past a membrane along a flow path extending from a source to a waste receptacle. This type of continuous flow may require hundreds of liters of dialysate fluid. More current dialysis systems recycle the exposed dialysate fluid rather than send it to a waste receptacle. Dialysis systems that recycle dialysate fluid have a dialysate fluid volume on the order of 6-10 liters.
In order to recycle dialysate fluid, urea and other blood waste compounds must be removed before the fluid is again passed by the membrane. One way to accomplish the removal of urea, for example, is to expose the urea to urease, which breaks the urea molecules down into ammonium ions and carbonate. The ammonium ions, which are toxic and should not be exposed to the membrane, can be bound, for example, by zirconium phosphate. In this case, zirconium phosphate acts as an ion exchanger and exchanges ammonium ions for sodium ions.
Because sodium is released by commercially available systems in the urea removal process, the sodium ion concentration in the dialysate fluid rises during the course of a dialysis treatment. The 6-10 liter systems have the capacity to dilute rising sodium ion levels such that the sodium ion concentration does not reach physiologically unacceptable levels. These 6-10 liter systems have the added benefit of being transportable relative to continuous flow type systems that require a source of high purity water used to make dialysate fluid in high volumes. For example, with the proper training and supervision, 6-10 liter systems can be easily adapted for use in a home environment without extensive equipment installation or home modification.
Another effect of dialysis treatment is the removal of excess water from the blood. Dialysis is often required because a patient's renal function is reduced, which means, in part, that the patient's ability to remove water from the blood is similarly reduced. In 6-10 liter systems, the total hydraulic fluid volume is expandable. Because the total volume is expandable, water can be removed from the patient into the dialysate system during dialysis. Water transport can be accomplished by creating a pressure differential between the blood side and the dialysate side such that water will flow from the blood side across the membrane to the dialysate side in a controlled manner.